What is the Mempool?
The mempool is the queue of pending Bitcoin transactions waiting to be included in a block. Short for "memory pool." Every node maintains its own. When miners assemble the next block, they pick from those pending transactions, prioritizing the highest fees per virtual byte.
Why It Matters
The mempool is where the fee market plays out in real time. Demand light: the mempool clears fast and a 1 sat/vB transaction confirms in the next block. Demand high: the mempool fills, fees climb, and low-fee transactions can sit unconfirmed for days. Sometimes weeks.
History gives the range. The May 2017 fee crisis pushed median fees above $30. January 2023 did it again with the Ordinals launch, when inscriptions and monetary transfers competed for block space and the mempool stayed clogged for months. April 2024 halving block. Fees briefly hit over $100 as bidders fought for the historic block reward.
Watching the mempool tells you when to send and when to wait. Nearly empty? Cheap and fast. Backlogged? Pay up or queue. You can track it live at mempool.space or on the network panel of Bitcoin Pulse.
How It Works
When you broadcast a Bitcoin transaction, it propagates to nodes across the network. Each node validates it (checking that the inputs exist and the signatures are valid) and adds it to its own local mempool. Miners building the next block pull from their mempool, ordering transactions by fee rate, and pack as many as fit within the block weight limit.
Once a transaction lands in a block, it leaves the mempool. The fee rate it paid becomes a data point in the broader market. If a block fills up and your transaction doesn't make it, you wait for the next one. Or you can use Replace-by-Fee (RBF) to rebroadcast the same transaction at a higher fee, and most modern wallets do this automatically when fee conditions change so you don't have to babysit a stuck transaction through a multi-day fee spike.